1. Field of the Invention
The present invention relates to focusing apparatuses and methods for controlling the same, and more particularly, to a focusing technique using phase-detection AF on an imaging surface.
2. Description of the Related Art
To take an image while viewing a live view (LV) screen (hereinafter referred to as “live view shooting”) has in recent years become more common on image pickup apparatuses, such as representatively a single-lens reflex camera. In particular, there has been a demand for comfortable shooting of a moving image while viewing the LV screen.
Most common automatic focusing (autofocus: AF) methods for image pickup apparatuses are the phase-difference detection method and the contrast detection method.
In the phase-difference detection method, light fluxes from a subject that have been passed through different exit pupil areas in an imaging optical system are caused to form images on a pair of line sensors to obtain a pair of image signals, and the defocus amount of the imaging optical system is obtained based on a phase difference between the pair of image signals. Thereafter, if a focusing lens is moved by an amount corresponding to the defocus amount, the imaging optical system is correctly focused on the subject (Japanese Patent Laid-Open No. 09-054242). However, in the typical structure in which the optical path to the image sensor is interrupted when light fluxes are caused to form an image on the phase-difference detection line sensors, the focusing process of the phase-difference detection method cannot be performed while performing LV shooting.
On the other hand, in the contrast detection method, an in-focus state is achieved by searching for a focusing lens position that produces a highest contrast evaluation value generated based on a picked-up image signal obtained using an image sensor (see Japanese Patent Laid-Open No. 2001-004914). The contrast detection method performs focusing based on a picked-up image signal and therefore is suitable for AF during LV shooting, and has in recent years been the mainstream AF method for LV shooting. However, the contrast detection method cannot easily judge a position or a direction to or in which the focusing lens is to be moved so that a subject is in focus. Therefore, in the contrast detection method, it may take time to focus, or the focusing lens may be moved in the wrong direction or past the in-focus position. In particular, when a moving image is shot, an image that is shot during movement of the focusing lens is also recorded, and therefore, the time and behavior of movement of the focusing lens until the focusing lens reaches the in-focus position affect the image quality of the moving image.
Therefore, for the AF control during shooting of a moving image, a sophisticated focusing operation that does not much affect the recorded image has been demanded, rather than the focusing speed (responsiveness) that is the time it takes to comes into the in-focus state. Therefore, a phase-detection-on-imaging-surface method has been proposed as a method that allows for a sophisticated focusing operation even during LV shooting. The phase-detection-on-imaging-surface method is a method for obtaining a defocus amount based on a phase difference between a pair of signals obtained by image sensors.
As a technique of implementing the phase-detection-on-imaging-surface method, a technique has been proposed that detects a defocus amount at the same time of picking up an image, by performing pupil division on the imaging pixels of an image sensor using microlenses and then receiving light using a plurality of focus detection pixels. Japanese Patent Laid-Open No. 2001-083407 describes a structure in which a photodiode included in each pixel, to which light is condensed by a single microlens, is divided so the photodiodes receive light of different pupil planes of the imaging lens. As a result, focusing can be performed by the phase-detection-on-imaging-surface method based on the outputs of two photodiodes. If the phase-detection-on-imaging-surface method is used, a defocus amount can be calculated to find the direction and amount of movement of the focusing lens even during LV shooting, and therefore, the focusing lens can be moved at a high speed and in a sophisticated manner.
Japanese Patent Laid-Open No. 2012-088617 has proposed a technique of driving the focusing lens in a sophisticated manner using the phase-detection-on-imaging-surface method during shooting of a moving image, in which the focusing lens is driven using a defocus amount that is corrected to have a smaller absolute value based on the degree of variations in the calculated defocus amount.
As in the image pickup apparatus described in Japanese Patent Laid-Open No. 2012-088617 cited supra, by performing an AF control while correcting and reducing the amount of drive of the focusing lens during shooting of a moving image, the focusing lens can be prevented from being moved past the in-focus position, and a sophisticated focusing lens drive can be achieved. However, the calculation of the defocus amount needs to be performed a number of times in order to calculate the degree of variations in the defocus amount, and therefore, a time lag may occur before a defocus correction amount is obtained.
Japanese Patent Laid-Open No. 2010-139942 has proposed a technique of driving the focusing lens based on the defocus amount calculated by the phase-detection-on-imaging-surface method when the reliability of the defocus amount is good, and based on the contrast evaluation value when the reliability is poor.
The pair of signals obtained by the phase-detection-on-imaging-surface method are not much separated from each other compared to the signals output by the dedicated line sensors used in the phase-difference detection method. In particular, there are characteristics that the accuracy of defocus detection is low when the defocus amount is large, i.e., there is a significant blur. Therefore, if the defocus amount is corrected based on the detected defocus amount as in the image pickup apparatus of Japanese Patent Laid-Open No. 2012-088617 cited supra, variations in the defocus amount may not be correctly detected, particularly when there is a significant blur. In such a case, the accuracy of the correction amount of the defocus amount decreases, and the drive performance of the focusing lens decreases due to overcorrection or insufficient correction.
If the autofocus method is switched between the phase-detection-on-imaging-surface method and the contrast method, depending on the reliability of the defocus amount calculated by the phase-detection-on-imaging-surface method as in Japanese Patent Laid-Open No. 2010-139942 cited supra, the opportunity to perform accurate focusing can be increased even when the reliability of the defocus amount calculated by the phase-detection-on-imaging-surface method is low.
However, in order to switch the autofocus method between the phase-detection-on-imaging-surface method and the contrast method, it is necessary to produce evaluation values that are used in the respective methods, and therefore, the number of necessary circuits or the processing load in the camera increases. Moreover, because a plurality of methods are changed, the process is more complicated, and the time and effort required for management or development are likely to increase. Also, as described above, when focusing is performed using the contrast method during shooting of a moving image, then if the controllability is poor (e.g., particularly the flexibility of setting of the drive speed of the focusing lens is low, etc.), the focusing operation is likely to affect image quality.